Phosphonium salts and methods of their preparation

ABSTRACT

Methods for preparing phosphonium salts by reacting a primary phosphine or a secondary phosphine with an ester compound selected from the group consisting of: a phosphate triester; a phosphonate diester; a sulfate diester; and a sulfonate ester; to form a phosphonium salt of formula VII 
                         
wherein each of R Q , R X , R Y , and R Z  is independently hydrocarbyl and X −  is a phosphate, phosphonate, sulfate, or sulfonate are provided herein. These phosphonium salts may find utility in a wide range of applications, including as surfactants, as polar solvents (ionic liquids), as antimicrobial agents, and as a component of spinning finish in polyamide fiber processing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 10/549,223, filed Feb. 8, 2007 (allowed), which is the U.S.National Phase application of International Application No.PCT/US2004/006961, filed Mar. 8, 2004, which claims benefit of priorityfrom Canadian Patent Application No. 2,424,215, filed Mar. 31, 2003,each of which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to phosphonium salts and their methods ofpreparation.

BACKGROUND OF THE INVENTION

Phosphonium salts may be used in a wide range of applications,including: as surfactants; as a component in spinning finish agents foraromatic polyamide fibers (JP11172577 A2, published Jun. 29, 1999); asantimicrobial agents (Kanazawa et al. (1994) Antimicrobial Agents andChemotherapy, vol. 38(5), p. 945-952); and as polar solvents known as“ionic liquids” (for a recent review, see Thomas Welton (Chem. Rev.1999, 99, 2071-2083)).

For many purposes, a phosphonium salt having a non-halide anion isdesirable. Phosphonium non-halide salts can be prepared by aconventional two-step process, comprising the steps of (a) reacting atertiary phosphine with an alkylhalide to obtain a quaternaryphosphonium halide salt, and (b) exchanging the halide anion with asuitable anion (by ion exchange or metathesis) to generate a quaternaryphosphonium salt having a non-halide anion.

However, this two-step process has several drawbacks. For example, thealkylhalides used to quaternize the tertiary phosphine are expensive andsome are corrosive and difficult to prepare and use. Also, the tertiaryphosphine may be expensive or difficult to make, the preparation thereofsometimes involving several steps and starting materials that areexpensive and pyrophoric (see for example Kanazawa et al., supra, andHugh R. Hays, J. Org. Chem. Vol. 31, pp. 3871-3820, which describemethods for preparing trimethylalkylphosphonium halides).

In addition, the two-step process generates large amounts of waste, assalt or acid by-products are usually removed by washing with water.Thus, the two-step process is inconvenient on an industrial scale.

Further, the end-product of the two-step process can be contaminatedwith residual halide ion, which may interfere with the intended utilityof the phosphonium salt. For instance, halide ions such as chloride ionscoordinate with group VII metals such as palladium and platinum and as aresult, the presence of chloride ion can interfere with the activity ofgroup VII metal catalysts. If a phosphonium salt is to be used in anenvironment where halide ions are unacceptable, even at low levels,halide salts should not be used in the starting materials or a furtherprocess must be used which ensures removal of halide ions from thephosphonium salt.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a method of preparing aphosphonium salt, the method comprising reacting a compound of formulaI:

wherein R¹ is hydrogen, R² is hydrogen or hydrocarbyl, and R³ ishydrocarbyl, with an ester compound defined by:

wherein each of R⁴, R⁵, R⁶, R⁷, R⁸, R⁹, and R¹⁰ is independentlyhydrocarbyl, to form a phosphonium salt of formula VII:

wherein R^(Q) is selected from R⁴ and R² when R² is hydrocarbyl, R^(X)is selected from R⁴ and R³, each of R^(Y) and R^(Z) is independently R⁴,and X⁻ is

Some of the compounds of formula VII that can be prepared by theforegoing process are novel. Thus, in another aspect, the inventionprovides a compound of formula VII:

wherein each of R^(Q), R^(X), R^(Y), and R^(Z) is independentlyhydrocarbyl; and X⁻ is

wherein each of R⁷, R⁸, and R⁹ is defined as above,with the provisos that:

when X⁻ is a phosphonate anion, then R^(Q), R^(X), R^(Y), and R^(Z) eachhas three or more carbon atoms;

when X⁻ is a sulfate then the sum of carbon atoms in R^(Q), R^(X),R^(Y), and R^(Z) is greater than 4; and

when X⁻ is methylsulfate, and one of R^(Q), R^(X), R^(Y), and R^(Z) ismethyl, the other of R^(Q), R^(X), R^(Y), and R^(Z) cannot all be2-cyanoethyl.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a ¹H-NMR (proton nuclear magnetic resonance) spectrum of amixture of cyclohexyltrimethylphosphonium dimethylphosphate anddimethylphosphoric acid.

FIG. 2 is a ³¹P-NMR of a mixture of cyclohexyltrimethylphosphoniumdimethylphosphate and dimethylphosphoric acid.

DETAILED DESCRIPTION OF THE INVENTION

In general, a phosphonium salt of formula VII can be prepared byreacting a phosphine of formula I (hereinafter referred to as the“starting phosphine”) with an ester compound selected from the groupconsisting of: a phosphate triester of formula II; a phosphonate diesterof formula III; a sulfate diester of formula IV; and a sulfonate esterof formula V. The overall reaction generates a quaternary phosphoniumsalt of formula VII and an acid counterpart of the ester (i.e.phosphoric acid, phosphonic acid, sulfuric acid, or sulfonic acid,respectively).

In one embodiment, the current method may be used for preparing aphosphonium of formula VII that has one or more methyl groups (i.e. one,two, three, or four methyl groups) attached to the phosphorus atom.

The current method may be especially suitable for preparing compounds offormula VII that are substantially free of halide ions.

In general, when the starting phosphine is a primary phosphine or asecondary phosphine, the ester is present in about three-fold ortwo-fold molar excess, respectively, relative to the starting phosphine,so as to provide roughly stoichiometric amounts of reagents.Specifically, when the starting phosphine is a primary phosphine (i.e.has a one hydrocarbyl group and two hydrogens attached to the phosphorusatom), the ester is present in about 3-fold molar excess of esterrelative to starting phosphine. When the starting phosphine is asecondary phosphine (i.e. has two hydrocarbyl groups and one hydrogenattached to the phosphorus atom), the ester is present in about 2-foldmolar excess of ester relative to starting phosphine. However in somecases, yields may be improved by using an excess of ester, for examplein the range of about 1.05 to about 3.0 fold excess relative to thestoichiometry of the overall reaction and preferably about 1.1 to about1.2 fold excess relative to the stoichiometric amount.

The temperature of the reaction is not critical and may range from aboutroom temperature to about 260° C. or higher, although lower temperatureswill result in longer reaction times. In general, the reaction proceedsreadily at elevated temperature, say between about 80° C. to about 220°C., preferably in the range of 100-190° C., and is often complete in 8hours at these temperatures.

However, as R⁴ groups on the ester increase in size (i.e. steric bulk),the reaction may become less efficient and higher temperatures or longerreaction times may be necessary to increase yield. Therefore, suitablevalues for R⁴ include but are not limited to: methyl, ethyl, n-propyl,isopropyl, n-butyl, iso-butyl, and tert-butyl. Certain esters, such asdimethyl-sulfate, are very active alkylating reagents and may be usedfor reactions carried out at moderate temperatures.

Also, the properties of the starting phosphine may affect the overallrate of the reaction. Secondary phosphines tend to be more reactive thanprimary phosphines. Thus, in general, reactions involving primaryphosphine are carried out at higher temperatures or for longer times orboth than are counterpart reactions involving secondary phosphines.

The starting phosphine can be added directly to an ester (a phosphatetriester, a phosphonate diester, a sulfate diester, or a sulfonateester), with stirring. However, the overall reaction is exothermic.Therefore, in order to control the temperature of the reaction mixture,it may be desirable to control the rate of addition in some cases andperhaps also to apply external cooling during the addition step. Inaddition, since alkylphosphines may be pyrophoric, it may be desirableto control the rate of addition of mono- or di-alkylphosphine so as toavoid having a large amount of unreacted mono- or di-alkylphosphinepresent in the reaction mixture, especially when the reaction is beingcarried out at elevated temperatures, for example over 100° C.

When the starting phosphine is a liquid at the temperature to be usedfor carrying out the reaction, the pressure of the reaction is notcritical, and the reaction may be conveniently carried out atatmospheric pressure, under an inert atmosphere, such as nitrogen. Someprimary and secondary phosphines that have short chain alkyl groups(such as dimethylphosphine) have low boiling points and may be gaseousand the temperature to be used for carrying out the reaction. When thestarting phosphine is a gas at the temperature to be used for carryingout the reaction, the reaction is suitably carried out under pressure(e.g. in an autoclave) under an inert atmosphere, such as nitrogen.

The reaction can be carried out in the absence of solvent, in order toavoid a further step of purifying product away from solvent. However,the reaction may also be carried out in the presence of a solvent. Insome cases, the presence of a solvent may be preferred as the solventmay enhance the rate at which the reaction proceeds.

If desired, any unreacted starting materials may be removed, forexample, by evaporating under vacuum.

The method of the invention produces a mixture of phosphonium salt andacid which may be used directly, for example as a solvent for chemicalreactions. Alternatively, the mixture of phosphonium salt and acid maybe subjected to purification steps to isolate the phosphonium salt. Forexample, dimethylphosphoric acid and methyl hydrogen sulfate can beremoved from the reaction mixture by evaporation, for example undervacuum at elevated temperatures (dimethylphosphoric acid decomposes at172-176° C., and methyl hydrogen sulfate decomposes at 130-140° C.; seeHandbook of Chemistry and Physics, 57^(th) Edition, CRC Press, Inc.,copyright 1976, pages C-435 and C-508). Alternatively, the acid productcan be removed by neutralizing the acid with a hydroxide of a Group IImetal (i.e. an alkaline earth metal hydroxide, such as calcium hydroxideor barium hydroxide) to form a precipitate and recovering theprecipitate by suitable means, such as filtration. Of note, calciumdimethylphosphate, a chemical that finds utility in polyester fibreprocessing (JP2001164461), can be prepared by the foregoing process. Ifthe phosphonium salt forms a two-phase system when mixed with water, itmay be possible to remove acid by washing the phosphonium salt withwater. Other purification processes known in the art, such aschromatography, can also be used to purify the phosphonium salt from thereaction mixture.

Suitable hydrocarbyl groups for R^(Q), R^(X), R^(Y), R^(Z), R², R³, R⁴,R⁵, R⁶, R⁷, R⁸, R⁹, and R¹⁰ include: substituted or unsubstituted C₁-C₃₀alkyl, substituted or unsubstituted C₃-C₈ cycloalkyl, substituted orunsubstituted C₂-C₃₀ alkenyl, substituted or unsubstituted C₂-C₃₀alkynyl, substituted or unsubstituted C₆-C₁₈ aryl, or substituted orunsubstituted C₇-C₃₅ aralkyl, although hydrocarbyl groups with not morethan 20 carbon atoms are preferred. R² and R³ together with thephosphorus atom to which R² and R³ are bonded can form a five- toeight-membered heterocycle or a heterobicycle, such as9-phosphabicyclo[3.3.1]nonane. It is noted that R² and R³ can beperfluoroalkyl. It is possible for the R groups (R^(Q), R^(X), R^(Y),R^(Z), R² and R³ when not perfluoroalkyl and R⁴ to R¹⁰) to bearsubstituents, or to include heteroatoms, provided that the substituentsor heteroatoms do not interfere with the preparation of the compounds ofthe invention, and do not adversely affect the desired properties of thecompound. Acceptable substituents may include alkoxy, halo, carboxy, andacetyl, and heteroatoms that may be acceptable include nitrogen, oxygenand sulphur. Substituents are likely to increase the cost of thecompounds of the invention and as the compounds are often used inindustrial applications (as solvents, surfactants, etc.), they are usedin such volume that cost is a significant factor. Hence, it iscontemplated that, for the most part, substituents will not be present,except for compounds in which one or more of R² and R³ isperfluoroalkyl. If necessary, one of skill in the art can readilydetermine whether substituents or heteratoms of the hydrocarbyl groupsinterfere with preparation or desired properties of the compounds byroutine experimentation that does not involve the exercise of anyinventive faculty.

In many cases, R^(Q), R^(X), R^(Y), R^(Z), R², R³, R⁴, R⁵, R⁶, R⁷, R⁸,R⁹, and R¹⁰ will be substituted or unsubstituted alkyl groups of 1 to 20carbon atoms. Thus, specific examples of values for R^(Q), R^(X), R^(Y),R^(Z), R², R³, R⁴, R⁵, R⁶, R⁷, R⁸, R⁹, and R¹⁰ include: methyl, ethyl,n-propyl, isopropyl, n-butyl, iso-butyl, tert-butyl, n-pentyl,cyclopentyl, iso-pentyl, n-hexyl, cyclohexyl, norbornyl, 3-methylphenyl(2,4,4′-trimethyl)pentyl, cyclooctyl, tetradecyl, etc. R² and R³ canalso be trifluoromethyl.

Mention is made of the following examples of values for R² and R³:methyl, trifluoromethyl, ethyl, n-propyl, iso-propyl, n-butyl,iso-butyl, tert-butyl, cyclopentyl, cyclohexyl, and norbornyl, and thecase where R² and R³ together with the phosphorus atom to R² and R³ arebonded form 9-phosphabicyclo[3.3.1]nonyl.

Mention is made of the following examples of values for R⁴, R⁵, R⁶, R⁷,R⁸, R⁹, and R¹⁰: methyl, ethyl, n-propyl, isopropyl, n-butyl, iso-butyl,and tert-butyl. It is noted that R¹⁰ can also be p-toluenyl. Examples ofsuitable esters for use in the inventive method include but are notlimited to: trimethylphosphate, dimethylsulfate,dimethylmethanephosphonate, and methyltosylate.

Phosphonium cations in which R^(Q), R^(X), R^(Y), and R^(Z) are notidentical are referred to as “asymmetric”. In some cases, it is desiredthat R^(Q), R^(X), R^(Y), and R^(Z) shall not be identical and inparticular, that at least one of R^(Q), R^(X), R^(Y), and R^(Z) shallcontain a significantly higher number of carbon atoms (for example 14 to20 carbon atoms) than the others of R^(Q), R^(X), R^(Y), and R^(Z).

For some applications, it is desired that at least one of R^(Q), R^(X),R^(Y), and R^(Z) shall contain a low number of carbon atoms (for example1 to 3 carbon atoms, more preferably 2 carbon atoms, and even morepreferably one carbon atom). For example, one, two, three or all ofR^(Q), R^(X), R^(Y), and R^(Z) can be methyl. Phosphonium salts with alow carbon content, say between 5 to 12 carbon atoms, may find utilityas ionic liquids or electrolytes in cases where a high ratio of chargeto molecular weight is required.

Phosphonium salts that may be used as surfactants include those in whichthree of R^(Q), R^(X), R^(Y), and R^(Z) are each independently methyl orethyl, preferably methyl, and the other is a saturated hydrocarbylhaving an unbranched chain of a higher number of carbon atoms, say 12 to30 carbon atoms, more preferably 12 to 20 carbon atoms. By a“surfactant” we mean a surface-active agent that reduces surface tensionwhen dissolved in water or water solutions, or that reduces interfacialtension between two liquids, or between a liquid and a solid.Surfactants include detergents, wetting agents, and emulsifiers.Surfactants may form micelles. The hydrocarbyl chain on the phosphoniummay bear substituents that do interfere with intended utility of thecompound as a surfactant, including but not limited to fluoro.

In some cases, it is preferred that at least one of R^(Q), R^(X), R^(Y),R^(Z) or R⁵ to R¹⁰ contains a higher number of carbon atoms, for example14 or more. For example, the presence of one or more long alkyl chainsmay increase the ability of a phosphonium salt to dissolve nonpolarorganic compounds. In addition, the presence of one or more long alkylchains may render the phosphonium salt “water immiscible”.

Compounds according to formula VII that are hydrophobic or “waterimmiscible” are preferred for some purposes. The term “water immiscible”is intended to describe compounds that form a two phase system whenmixed with water but does not exclude compounds that dissolve in waternor compounds that will dissolve water, provided that the two phasesystem forms. Water immiscibility is a desirable feature of aphosphonium salt not only because it renders the compound useful as asolvent for biphasic reactions with an aqueous phase, but also becauseit facilitates purification and isolation of the phosphonium salt whenprepared according to certain methods. By way of illustration, when themethod of the invention produces a water-immiscible phosphonium salt andan acid, the acid can be removed from the reaction products by washingthe phosphonium salt with water. Compounds of formula VII that have alarge total number of carbons, say equal to or greater than 20 and inparticular greater than 25 or 26, or have at least one aryl group aremore hydrophobic. There is no critical upper limit on the total numberof carbon atoms that may be present in a compound of formula VII.However, it is unlikely that the total will exceed 50.

If the compound of formula VII is intended for use as a solvent, then ingeneral, it is preferred that the compound is a liquid below 100° C.,more preferably below 50° C., and most preferably at or below roomtemperature. Values for R^(Q), R^(X), R^(Y), R^(Z), R², R³, R⁴, R⁵, R⁶,R⁷, R⁸, R⁹, and R¹⁰ can be selected to yield compounds that are liquidat room temperature. Increasing the total number of carbon atoms presentin the hydrocarbyl groups R^(Q), R^(X), R^(Y), R^(Z), and R² to R¹⁰ willtend to increase the melting point, although this effect can becounteracted somewhat by asymmetry, branching of the hydrocarbyl groupsR^(Q), R^(X), R^(Y), R^(Z), and R² to R¹⁰, and the tendency ofsterically bulky ions to coordinate poorly. Specifically, the meltingpoint tends to decrease as the degree of asymmetry around the phosphorusatom increases. Also, the melting point of the salt will tend todecrease as the degree of branching of the hydrocarbyl groups R^(Q),R^(X), R^(Y), R^(Z), and R² to R¹⁰ increases. Branching can occur at thealpha or omega carbon or at any intermediate point. In addition, themelting point of the salt will tend to decrease as steric bulk increasesaround either or both of the phosphorus atom of the cation and thecentral atom of the anion (the sulfur atom or phosphorus atom or carbonatom); for this reason, it may be preferred for one or more of R groupson either or both of the cation and anion (i.e. R^(Q), R^(X), R^(Y),R^(Z), R⁵, R⁶, R⁷, R⁸, R⁹, and R¹⁰) to have three or more carbon atoms.

Thus, the current invention contemplates compounds of formula VII whereproperties may be modified by varying the values of the R groups presenton either the anion or the cation. Selection of particular values forR^(Q), R^(X), R^(Y), R^(Z), and R² to R¹⁰ to achieve particular meltingpoints, degrees of water immiscibility, or surfactant properties iswithin the competence of a person skilled in the art, although it mayrequire some routine experimentation.

Compounds according to formula VII that have chirality provide a chiralenvironment for chemical reactions and may be especially suitable forcertain purposes, such as a reaction having an asymmetric or chiraltransition state that can be stabilized by interaction with a suitablesolvent. Examples of chiral compounds of formula VII include compoundscontaining a phosphonium cation wherein R^(Q), R^(X), R^(Y), and R^(Z),are all different or wherein one of R^(Q), R^(X), R^(Y), and R^(Z), isan enantiomer, such as 2,4,4′-trimethylpentyl, which group has onechiral atom.

Mention is made of the following examples of compounds of formula VII:

cyclohexyltrimethylphosphonium dimethylphosphate;

dibutyldimethylphosphonium dimethylphosphate;

dicyclohexyldimethylphosphonium dimethylphosphate; and

diisobutyldimethylphosphonium dimethylphosphate.

Phosphonium salts described herein may find utility in a wide range ofapplications. For example, phosphonium salts in which three of R^(Q),R^(X), R^(Y), and R^(Z) are methyl and the other is a saturated orunsaturated hydrocarbyl having an unbranched chain of a higher number ofcarbon atoms, say 12 to 30 carbon atoms, may find utility asantimicrobial agents (Kanazawa, supra) or surfactants. Phosphoniumphosphates may find utility as a component of spinning finish(JP11172577). The phosphonium salts of the current invention may also beused as polar solvents known as “ionic liquids” for chemical reactionssuch as Michael additions, aryl coupling, Diels-Alder, alkylation,biphasic catalysis, Heck reactions, hydrogenation, or for enzymaticreactions, for example lipase reactions (for a recent review of ionicliquids, see Thomas Welton (Chem. Rev. 1999, 99, 2071-2083)).

EXAMPLES

In the following examples, starting material phosphines are made byCytec Canada, Inc. and their purity determined by gas chromatography(GC). The remaining starting materials were purchased from Aldrich andused as they were purchased. Structures were confirmed by NMR (nuclearmagnetic resonance spectrometry) and by FAB MS (Fast Atom Bombardmentmass spectrometry), as indicated.

Example 1 Preparation of CyclohexyltrimethylphosphoniumDimethylphosphate

Cyclohexylphosphine (14.5 g, 98%, 0.1255 mole) was added by drippingthrough an addition funnel over a period of 10 minutes to a flaskcontaining trimethylphosphate (95 g, 97%, 0.6578 mole, b.p. 197° C.)preheated to 140° C. under nitrogen, with stirring. There was no suddenchange in temperature associated with addition. The mixture was heatedto reflux (about 165° C.).

As the reaction proceeded, the temperature of the mixture graduallyincreased to 210° C. and was maintained at this temperature withstirring for 15 minutes. The total reaction time was about 8 hours.

The reaction mixture was then cooled and decanted into a flask. Excesstrimethylphosphate was removed from the reaction mixture by heating to180° C. under vacuum 5 mm Hg.

The product was a glassy, colourless liquid with a pH of about 2-3 pHunits. The presence of cyclohexyltrimethylphosphonium dimethylphosphateand dimethylphosphoric acid was confirmed by ¹H (see FIG. 1), ¹³C, and³¹P (see FIG. 2) NMR and FAB MS analysis. ¹H-NMR (CDCl₃, 300.13 Hz, δ)signals for the characteristic methyl groups are: 1.96 (d, J=14.2 Hz,P—CH₃), 3.62 (d, J=10.8 Hz, O═P—O—CH₃); ³¹P-NMR (CDCl₃, 81.015 Hz, δ):30.60 (P⁺), 1.63 (O═P—O—CH₃).

Example 2 Preparation of Dibutyldimethylphosphonium Dimethylphosphate

A 500 ml 2 neck round-bottomed flask fitted with a condenser was chargedwith 108.0 g (0.77 mole) trimethylphosphate and heated to 135° C. undernitrogen with stirring. Di-n-butylphosphine (93.2 g, 0.64 mole) wasadded to the flask over a period of 8 hours. The temperature of thecontents of the flask increased to 155° C. during the addition of afirst 6.6 g of the total 93.2 g of di-n-butylphosphine. The reaction wasmaintained at 150° C. for 2 hours.

Following the incubation period, unreacted trimethylphosphate wasremoved by evaporation for 6 hours at 100° C. under reduced pressure (20mmHg).

A colourless and viscous liquid (180.7 g) was obtained. ³¹P, ¹³C and ¹HNMR and FAB MS confirmed the presence of dibutylphosphoniumdimethylphosphate and dimethylphosphoric acid. ¹H-NMR (CDCl₃, 300.13 Hz,δ) signals for the characteristic methyl groups are: 1.65 (d, J=13.8 Hz,P—CH₃), 3.20 (d, J=10.6 Hz, O═P—O—CH₃); ³¹P-NMR (CDCl₃, 81.015 Hz, δ):30.36 (P⁺), 2.06 (O═P—O—CH₃).

Example 3 Preparation of DicyclohexyldimethylphosphoniumDimethylphosphate

A round-bottomed flask fitted with a condenser was charged with 280.3 g(1.9 mole) trimethylphosphate and heated to 100° C. under nitrogen withstirring. Dicyclohexylphosphine (277.2 g, 1.4 mol) was gradually addedto the flask over a period of 7.5 hour. The reaction was exothermic,with the temperature of the contents of the flask rising rapidly to150-160° C. during the addition of dicyclohexylphosphine. The reactionmixture was maintained at a temperature of 150-160° C. for 2 hours, thencooled to room temperature.

Upon cooling to room temperature, the product crystallized into a clear,colourless solid. Unreacted trimethylphosphate was removed byevaporation under reduced pressure (5 mmHg) at 170° C. for 12 hour.

Analysis by ³¹P and ¹H NMR and MS confirmed the presence ofdicyclohexyldimethylphosphonium dimethylphosphate and dimethylphosphoricacid. ¹H-NMR (CDCl₃, 300.13 Hz, δ) signals for the characteristic methylgroups are: 1.76 (d, J=13. Hz, P—CH₃), 3.39 (d, J=10.6 Hz, O═P—O—CH₃);³¹P-NMR (CDCl₃, 81.015 Hz, δ): 34.48 (P⁺), 2.28 (O═P—O—CH₃).

Example 4 Preparation of Diisobutyldimethylphosphonium Dimethylphosphate

A round-bottomed flask was charged with 136.0 g (0.96 mole)trimethylphosphate and heated to 140° C. under nitrogen with stirring.While stirring vigorously, a solution of 70.3 g (0.48 mol)diisobutylphosphonium and 11.0 g of dimethylcarbonate were added to theflask over a period of 2.5 hours at 135° C. The reaction was exothermicduring the addition of the phosphine. The reaction mixture wasmaintained at 135° C. for a total of 8 hours, then cooled roomtemperature. ³¹P, ¹H, and ¹³C NMR analyses confirmed the presence of thediisobutyldimethylphosphonium dimethylphosphate and dimethylphosphoricacid.

We claim:
 1. A method of preparing a phosphonium salt, the methodcomprising: reacting a compound of formula I:

wherein R¹ is hydrogen, R² is hydrogen or hydrocarbyl, and R³ ishydrocarbyl, with an ester compound defined by:

wherein each of R⁴, R⁷ and R⁸ is independently hydrocarbyl, to form aphosphonium salt of formula VII:

wherein R^(Q) is selected from R⁴ and R² when R² is hydrocarbyl, R^(X)is selected from R⁴ and R³, each of R^(Y) and R^(Z) is independently R⁴,and X⁻ is


2. The method of claim 1, wherein R² is hydrogen and R³ is ahydrocarbyl.
 3. The method of claim 1, wherein each of R² and R³ areboth hydrocarbyl.
 4. The method of claim 1, wherein the hydrocarbyl is asubstituted or unsubstituted alkyl group of 1 to 20 carbon atoms.
 5. Themethod of claim 4, wherein the hydrocarbyl is selected from the groupconsisting of: methyl, trifluoromethyl, ethyl, n-propyl, iso-propyl,n-butyl, iso-butyl, tert-butyl, cyclopentyl, cyclohexyl, and norbornyl.6. The method of claim 3, wherein R² and R³ together with the phosphorusatom to which R² and R³ are bonded form a five- to eight-memberedheterocycle or heterobicycle.
 7. The method of claim 6, wherein theheterobicycle is 9-phosphabicyclo[3.3.1]nonyl.
 8. The method of claim 1,wherein R⁴ is selected from the group consisting of: methyl, ethyl,n-propyl, iso-propyl, n-butyl, iso-butyl, and tert-butyl.
 9. The methodof claim 8, wherein R⁴ is methyl.
 10. The method of claim 1, wherein thephosphonium salt has a total number of carbon atoms between 20 and 50.11. The method of claim 10, wherein the phosphonium salt has a totalnumber of carbon atoms between 25 and
 50. 12. The method of claim 1,wherein the phosphonium salt is water immiscible.
 13. The method ofclaim 1, wherein the phosphonate ester is dimethylmethanephosphonate.